Bone-density measuring instrument

ABSTRACT

A bone density measuring instrument including a scanning assembly having means for transmitting a radiation beam through a selected bone section. The attenuated radiation beam transmitted through the bone section produces an electrical output signal representing the intensity of the detected beam. The scanning assembly is moved along a predetermined path relative to the bone to scan a selected section of the bone, with the electrical signal varying in accordance with variations in the density of the scanned bone section. The scanning movement is automatically stopped at a preselected stop position, which is adjustable, and the scanning assembly is then automatically returned to a start position, which is also adjustable. Thus the scanning movement of the scanning assembly can be limited to the selected bone section. Means are provided for returning the scanning assembly from the stop position to the start position at a rate substantially faster than the scanning rate. The electrical pulses representing the detected beam intensity are fed into a scaler which counts the pulses, and the counting operation is periodically interrupted for a brief interval to permit the accumulated count to be transferred into a buffer storage. The counting is then resumed, while the data is read out of the buffer storage means at the relatively slow rate required by the output mechanism, such as a printer, tape punch, or the like. A radiograph template is also disposed which permits two different X-rays to be made of the same bone, from two different positions, on a single radiograph film. One radiograph is used to locate the selected bone section on a screen on the front of the instrument, where an illuminated dot indicates the position of the scanning beam, and the other radiograph permits measurement of the thickness of the particular bone section that is canned to permit calculation of the bone density. Various adapters are provided for receiving different body extremities in the instrument, so that a number of different bones can be scanned in the same instrument.

tlite ttcs W et al. a 1 Web. l, 1972 [54] ONE-DENSITY MEASURING beam transmitted through the bone section produces an elec- INSTRUMENT trical output signal representing the intensity of the detected beam. The scanning assembly is moved along a predetermined 2 Inventors: Robert Olslms Glen Ellyn; lnseph path relative to the bone to scan a selected section of the NaPel'villey both bone, with the electrical signal varying in accordance with variations in the density of the scanned bone section. The scanning movement is automatically stopped at a preselected stop position, which is adjustable, and the scanning assembly [22] Filed: Apr. 2, 1969 Y i is then automatically returned to a start position, which is also ad'ustable. Thus the scannin movement of the scannin as- [21] Appl 812657 sei nbly can be limited to the s elected bone section. Mean: are provided for returning the scanning assembly from the stop [73] Assignee: Packard Instrument Company, Inc.,

Downers Grove, Ill.

52 us. Cl ..250/83;3 0, 250/715 s Position to the Start Position at a rate substantially faster than 5 the scanning rate. The electrical pulses representing the de- [58] Field of Search ..250/71.5 s, 83.3 D meted beam intensity are fed a scale which mums pulses, and the counting operation is periodically interrupted [56] References Cited for a brief interval to permit the accumulated count to be transferred into a buffer storage. The counting is then UNITE TAT S PATENTS resumed, while the data is read out of the buffer storage means at the relatively slow rate required by the output mechanism,

Madigan such as a printer tape punch or {he A radiograph tem. 3,057,998 10/1962 west r -250/71-5 X plate is also disposed which permits two different X-rays to be 3,244,881 4/1966 Hansen et al. .....250/83.3 made of the same bone, from two different positions, on a sin- 3,344,275 9/ i967 Marcaalet al. ..250/83.3 X gle radiograph film One radiograph is used to locate the selected bone section on a screen on the front of the instru- Primary Examiner-James W. Lawrence ment, where an illuminated dot indicates the position of the Assistant Examiner-D. L. Willis scanning beam, and the other radiograph permits measure- Attorneywolfe, Hubbard, Leydig, Voit & Osann ment of the thickness of the particular bone section that is I canned to permit calculation of the bone density. Various [57] ABSTRACT adapters are provided for receiving different body extremities in the instrument, so that a number of different bones can be A bone density measuring instrument including a scanning as- Scanned in the same instrument sembly having means for transmitting a radiation beam through a selected bone section. The attenuated radiation 10 Claims, 14 Drawing Figures PATENTED FEB 1 m2 SHEET 2 UF 8 PATENTEU FEB 1 m2 SHEET '4 BF 8 m w W WWW M r W2 7% l) M a M BONE-DENSITY MEASURING INSTRUMENT The present invention relates generally to instruments for making in vivo measurements of bone density and, more particularly, to instruments which make such measurements by measuring the transmission of a radiation beam through the bone.

Nondestructive in vivo measurements of the density of bones in either humans or animals provide information which is important for a number of purposes. For example, the density measurements provide an indication of the amount of mineral contained in the bone, and thus are useful in diagnosing and treating demineralizing diseases such as osteoporosis and osteomalacia. Such measurements are also useful in determining the effect of a sustained lack of stress on the bones, due to weightless space flights or extended bed rest for example, or the effect of diet, pregnancy, or certain drugs such as fluorides, steroids, cortisone-type drugs and the like.

When bone density measurements are made by measuring the transmission of a radiation beam through the bone, the absorption coefficient of the bone, designated [L3, is determined and then correlated with bone density. More specifically, the greater the transmission of the radiation beam through the bone, the lower the bone density and, therefore, the lower the mineral content. Of course, the radiation transmission is a function not only of the bone density but also the bone thickness, and thus the bone thickness must also be measured and taken into account. The equation for the transmitted intensity (l) for mcnoenergetic narrow beam radiation is} where 1,, =original radiation exposure rate p the linear absorption coefficient of the soft tissue surrounding the bone, (cm.")

y. the linear absorption coefficient of the bone, (cm.") I attenuated radiation exposure rate In [/1 }L ]J. b)

the thickness of the soft tissue through which the radiation beam is transmitted, (cm.)

b the thickness of the bone through which the radiation beam is transmitted. (cm.)

Solving the above equation for the absorption coefficient 0f the bone,

It is a primary object of the present invention to provide an improved bone densitometer which automatically scans a preselected bone section with a radiation beam, and provides the maximum amount of information on the density of the bone throughout the particular section that is scanned.

Another object of the invention is to provide an improved bone densitometer of the foregoing type which facilitates location of the scanning beam at the precise bone section selected for the density measurement.

A further object of the invention is to provide an improved bone densitometer of the type described above which reduces the time required for each scanning cycle, thereby permitting a greater number of measurements to be made with the instrument in any given time period.

A specific object of one particular aspect of the invention is to provide an improved radiograph template which permits two different radiographs of the selected bone to be made from two different positions on a single sheet of radiograph film, one of the radiographs being used to locate the particular section of bone to be scanned, and the other radiograph being used to determine the thickness of the particular bone section to be scanned.

Still another object of the invention is to provide an improved bone densitometer which can be used to measure the density of bones in a number of different parts of the body. In this connection, a particular object of the invention is to provide such an improvedbone densitometer which is capable of scanning along either a vertical or horizontal axis.

Other objects and advantages of the invention will be apparent from the following detailed description taken in con nection with the accompanying drawings, in which:

FIG. 1 is a perspective view of a bone densitometer embodying the invention;

FIG. 2 is an enlarged perspective of one particular adapter suitable for mounting in the bone densitometer of FIG. 1 for measuring the density of the os calcis;

FIG. 3 is an enlarged fragmentary representation of an os calcis radiograph with scales superimposed thereon to permit correlation with the screen on the front of the bone densitometer shown in FIG. 1;

FIG. 4 is a perspective of a radiograph template for use in making two different radiographs of a selected finger on a single sheet of a radiograph film;

FIG. 5 is a representation of a radiograph made with the use of the template shown in FIG. 4;

FIG. 6 is a perspective view of one of the instrument units shown in FIG. 1 having an adapter mounted thereon for scanning the phalanx in a selected finger;

FIG. 7 is a perspective view of one of the instrument units shown in FIG. 1 having an adapter mounted thereon for scanning a bone in the patients arm;

FIG. 8 is a perspective view of the internal mechanism contained in one of the instrument units shown in FIG. 1, and with certain elements shown in exploded positions;

FIG. 9 is an enlarged section taken along line 9-9 in FIG: 8;

FIG. 10 is an enlarged section taken along line 10-10 in FIG. 9;

FIG. 11 is an enlarged section taken along line 11-11 in FIG. 8;

FIGS. 12a and 12b, and 13 comprise a schematic circuit diagram of one portion of the electronic control system included in the bone densitometer shown in FIG. 1; and

While the invention is susceptible of various modifications and alternative forms, certain specific embodiments thereof have been shown by way of example in the drawings which will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Turning now to the drawings and referring first to FIG. 1, there is shown a bone-density measuring instrument comprising two units 10 and 11 interconnected by means of a cable 12. The top of the unit 10 forms a well 13 designed to receive various adapters for supporting different parts of the body, de-

pending upon the particular bone to be measured. The density of the particular bone of interest is measured by transmitting a radiation beam through a selected section of the bone as indicated on a screen 14 on the front of the unit 10. The instrument automatically scans across a selected section of the bone with the radiation beam, with the intensity of the transmitted or attenuated beam being continuously detected and measured by a system which automatically provides a printed output 15 emerging from the second unit 11; this printed output indicates the measured intensity of the attenuated radiation beam, thereby providing an indication of the bone density.

Turning to FIGS. 2 and 3 for a more detailed description of how the instrument shown in FIG. 1 is used to measure the density of the os calcis (heel bone), an adapter 16 is mounted in the well 13 to hold the patients foot in the position illustrated in FIGS. 2 and 3. More particularly, the adapter 16 includes a pair of vertical side members 17 and 18 connected to a pair of outwardly extending horizontal top members 19 and 20 which carry depending pins 21 and 22 designed to register with complementary holes formed in the top of the unit 10 on opposite sides of the well 13. The two side members 18 and 19 are provided with thin windows 17a and 18a to permit maximum transmission of a radiation beam through the heel portion of a foot supported on a pair of inclined bottom members 23 and 24, between the two side members 18 and 19. Since the adapter 16 is fixed in a predetermined position in the well 13 by means of the pins 21 and 22, and since the position of the patients heel within the adapter 16 is fixed by the configuration of the inclined bottom members 23 and 24, it can be seen that the adapter 16 assures that the particular bone to be scanned by the radiation beam i.e., the os calcis, is located between the two windows 17a and 18a.

In order to correlate the instrument screen 14 with the profile of the bone to be scanned within the well 13, a radiograph is made of the os calcis, with the patients foot positioned within the adapter 16, before the adapter is mounted in the well 13. Alternatively, of course, two similar adapters may be used, i.e., one mounted in the instrument, and another removed from the instrument for use in making the radiograph. When the radiograph is made, scales corresponding to the scales on the instrument screen 14 are superimposed on the radiograph to permit correlation of specific areas of the instrument screen with the corresponding specific portions of the bone profile shown by the radiograph. A representation of a typical foot radiograph, made with the patients foot positioned within the adapter 16 is shown in FIG. 3; it can be seen that scales identical to the scales on the screen 14 on the front of the instrument unit are superimposed over the os calcis 25, which is the particular bone through which the radiation beam is to be transmitted to determine the bone density. Consequently, the particular bone section to be scanned by the radiation beam in the instrument unit 10 can be quickly located on the instrument screen 14, by locating the numbers on the vertical and horizontal screen scales which correspond to the numbers of the bone section of interest on the radiograph. As will be described in more detail below, the location of the radiation beam is also indicated on the screen 14 by means of an illuminated dot, and thus the position of the beam relative to the bone section to be scanned is visible to the instrument operator on the screen 14.

Another bone that is often used in bone-density measurements is the phalanx (finger bone), typical radiographs of which are represented in FlG. 5. The particular phalanx that is most commonly used is in the third digit, and thus the scale that is superimposed on the radiograph 30 represented in FIG. 5 is located so that the second phalanx of the third digit lies approximately in the central areas of the two scales. These scales are, of course, used to locate the particular section of the phalanx to be scanned, in the same manner described above for the os calcis. In addition, however, FIG. 5 includes a second radiograph 31 of the same bone for use in determining the thickness of the particular bone section to be scanned; as explained previously, this thickness value is needed to determine the linear absorption coefficient. More particularly, the second radiograph 31 represents in FIG. 5 includes the same area of the finger within the scaled area ofthe first radiograph, but taken from the side of the finger so as to show the thickness ofthe phalanx to be scanned. As can be seen in FIG. 5, a scale is superimposed on the second radiograph 31 to permit correlation with the corresponding scale on the first radiograph, and on the instrument screen 14.

In accordance with one important aspect of the present invention, an improved radiograph template is provided for permitting two different radiographs to be made of the same body extremity, such as a finger for example, in two different positions on a single radiograph film. Thus, the radiograph template shown in FIG. 4 includes a base plate 40 made of a material which transmits X-rays for exposing a sheet of radiograph film in a standard film cassette received positioned within depending flanges 40a on the underside of the baseplate. Mounted on the upper surface of the plate 40, or formed as an integral part thereof, are a first receptacle 41 designed to receive a finger so as to locate the finger in a first predetermined position on the baseplate 40, and a second receptacle 42 for receiving the same finger and locating it in a second predetermined position on the baseplate 40. As illustrated in FlG. 4, the hand is placed flat on the baseplate 40 when the finger is placed in the first receptacle 41, so as to produce a radiograph corresponding to the radiograph 31 shown in FIG. 5. When the finger is placed in the receptacle 42, it is turned from its position in the first receptacle 42. so as to produce a radiograph corresponding to the radiograph 31 shown in FIG. 5. The scales shown in the exemplary radio graph of FIG. 5 are preferably formed by providing such scales, made of an X-ray shielding material, on the underside of the baseplate 40.

In keeping with the invention, an X-ray shield plate 43 is hinged to the baseplate 40 between the first and second receptacles 41 and 42, respectively, for the purpose of shielding the radiograph film under either one of the receptacles while the film under the other receptacle is being exposed. More particularly, the shield plate 43 is hinged to a rib or bar 44 on the upper surface of the baseplate 44, so that the hinged end of the plate 43 is positioned above the surface of the plate 40 by a distance about equal to the thickness of a normal hand. When the patients hand is placed in the receptacle 41 on the left-hand side of the template, the shield plate 43 is pivoted to the right so as to cover the second receptacle 43, resting against a pair of stop pins 45. Consequently, the portion of the underlying radiograph film lying to the right of the mounting bar 44 is shielded by the plate 43 while the portion ofthe film to the left of the bar 44 is exposed to form the radiograph 30. The patients hand is then removed from the receptacle 41, and the shield plate 43 is pivoted to the left so that it covers the receptacle 41, and the underlying film lying to the left of the mounting bar 44. The patients finger is then placed in the receptacle 42, and the right-hand portion of the radiograph film is exposed to form the radiograph 31, while the shield plate 44 protects the previously exposed portion of the film under the receptacle 41. Thus, two different radiographs of the same finger in two different positions are made on a single sheet of radiograph film.

In FIG. 6, there is illustrated an adapter 32 mounted in the well 13 of the unit 10 for holding the patients third digit in the proper position for scanning by the radiation beam. More particularly,the adapter 32 includes a base panel 33 adapted to fit along one ofthe sidewalls of the well 13, and connected to a supporting flange 34 adapted to rest against the exterior surface of the unit 10. As can be seen in FlG. 6, the unit 10 is preferably oriented with the instrument screen 14 on the top when the finger adapter 32 is utilized. To insure that the adapter 32 is properly positioned within the well 13, and to prevent sliding movement thereof, a pin 35 registers with the complementary hole formed in the exterior wall of the unit 10. The patients hand is placed palm down on the surface of the base panel 33 and advanced until the end of the third digit engages a positioning bar 36, with the thumb side of the hand engaging a positioning post 37. To insure that the finger to be scanned is held flat against the panel 33, and to locate the third digit in exactly the desired position, a locator assembly 38 slidably mounted on a pair of posts 38a and 38b is lowered down against the top of the patients finger. The base member 33 is provided with a window directly beneath the central portion of the locator assembly 38 to permit transmission of radiation beam through the second phalanx of the third digit. Since the adapter 32 is fixed in a predetermined position in the well 13 by means of the pin 35, and since the position of the patients finger within the adapter 32 is fixed by the positioning elements 36 and 37 and the locator assembly 38, it can be seen that the adapter 32 assures that the particular bone to be scanned by the radiation beam is located in the proper position.

In FIG. 7 there is shown an adapter 46 for positioning the patients arm within the instrument well 13 for scanning a selected bone in the arm. The adapter 46 includes a base panel 47 and a supporting flange 48 similar to the panel 33 and flange 34 described above in connection with the finger adapter 32. The flange 48 includes a pin 48a which registers with the complementary hole in the external wall of the instrument to position the adapter within the well 13.To receive the patient's arm, an open-ended box 49 having a hinged lid 49a is secured to the base panel 47, and a grip rod 49b is provided within the box 49 for locating the patients arm in a predetermined position when the bar 49b is gripped.

. While several exemplary adapters are described herein, it will be appreciated that any number of different adapters may be designed to receive different parts'of the body containing bones to be scanned. One of the significant features of the instrument provided by this invention is that-it is universally applicable to a wide variety of different adapters, so that bones located in many different parts of the body may be readily scanned to determine the density thereof.

Turning now to the internal system of the unit 10 for automatically scanning the'selected bone section with a radiation beam, and referring particularly to FIGS. 8 through 11, the radiation beam is generated by a radioactive source material S, contained within a housing 50 mounted on the front of a generally C-shaped frame 51 withinthe well 13. The housing 50 is made of a high-density material such as brass, and a removable plug 51 made of the same material encloses the source material within the housing 50.

In bone densitometry, it is generally preferred to measure transmitted radiation within the energyrange of about 30 to about 100 Kev. Lower energy radiation is attenuated by the flesh surrounding the bone as well as the bone itself, whereas higher energy radiation passes through the bone with little or no attenuation. Also, measurement ofmonoenergetic" radiation provides the most reliable indication of the effect of the bone on the transmission of the radiation beam. Accordingly, it is preferred to use a radioactive source material which emits a substantial percentage of its radiant energy within the 50 to 70 Kev. range In this connection, a particularly preferred radioactive source material for use in the present invention is Americium 241, which produces radiation that is about 85 percent in the 50 to 70 Kev. range, and also has a relatively long half life.

When it is desired to scan a bone placed within the instrument well 13, a shutter 52 is opened by energizing a solenoid SL1, so that the radiation emitted by the source material S is transmitted through a collimator 54 to produce a collimated radiation beam. When the shutter 52 is opened by energization of the solenoid SL1, a plunger 55 extending upwardly from the shutter 52 releases the actuator of a switch 56 to energize an indicator light L14 (FIG. 12a) mounted inside a count button B10 on the front of the unit 11, thereby illuminating the button B10 to indicate to the operator that the shutter is open and a scan is in progress. In addition, the switch 56 causes an indicator light L11 (FIG. 12a) within a stop button B11 on the front of the unit 11 to flash on and off in the event that the electronic control system indicates that the shutter should be closed, i.e., that a scan has been completed, but the switch 56 indicates that the shutter is still open; this is a safety feature that provides a signal to the operator in the event that the shutter mechanism fails to close because of a mechanical failure, for example, thereby continuing transmission of the radiation beam after completion of a scan. The operator can then take appropriate precautions to avoid extended exposure to the radiation beam until the shutter mechanism is repaired.

To provide a continuous indication of the position of the radioactive source S and the scanning assemblies associated therewith, an indicator light 57 mounted directly in front of the radioactive source S in a forwardly extending portion of the housing 50, is continuously energized. The illumination from the bulb 57 passes through an aperture formed in the forward end of a cylinder 58 telescoped within the forward end of the housing 50. The apertured forward end of the cylinder 58 bears against the rear surface of the screen 14 on the front of the instrument unit 10, so that an illuminated dot appears on the screen to provide a continuous indication of the exact position of the radioactive source S and the scanning assemblies associated therewith. In order to maintain the forward end of the cylinder 58 in continuous engagement with the rear surface of the screen 14 during the scanning movement of the housing 50 which carries the cylinder 58, and thereby provide a well-defined light spot on the screen, the cylinder 58 is biased against the rear surface of the screen 14 by a spring 59.

, As the radiation beam is transmitted through the particular body extremity placed within the instrument well 13. the resulting attenuated beam is detected by a collimated detector assembly 70 mounted on the rear of the frame 81 in precise alignment with the collimator 54 on the front of the unit. When the radioactive source material S is Americium 241, a preferred detector comprises a sodium iodide (thallium-activated) crystal contained within a housing 71. A collimator 72 extends forwardly from the housing 71 to receive the attenuated radiation beam and to transmit a well-collimated beam to the crystal contained within the housing 71. The collimator 72 is removably mounted within the rear wall of the frame 81 by means of a setscrew 73, and includes a flanged front portion 72a which fits against the surface of the frame 51 to position the collimator within the detector assembly. The flange 72a also serves as a shield to reduce background radiation reaching the detector crystal. To prevent light and moisture from entering the detector crystal, it is preferably covered on the entrance side by a 0.13-mm. beryllium foil, for example; this foil absorbs only a very small percentage of the radiation because of its small thickness and low atomic number.

The light pulses which result when the radiation beam interacts with the detector crystal within the housing 71 are picked up by a photomultiplier 80, which in turn produces an electrical output signal representing the intensity of the light pulses produced by the detector crystal and, therefore, the in tensity of the attenuated radiation beam. These electrical output signals from the photomultiplier are supplied through a preamplifier to an electronic control system to be described below, for automatically determining the intensity of the radiation beam transmitted through successive areas of the particular bone section scanned by the radiation beam. As explained previously, these intensity measurements can then be used to determine the density of the scanned bone section.

As can be seen most clearly. in FIG. 8, the U-shaped frame 81, which carries both the source assembly and the detector assembly 70, is mounted for movementalong either an X-axis or a Y-axis i.e., either vertically or horizontally as shown in FIG. 8. More particularly, when it is desired to move the source and detector assemblies (referred to collectively as the scanning assembly) along the X-axis defined by a guide rod 96, a reversible stepping motor 82 drives a belt 83 which in turn drives a ball screw 84. The motor 82 and oneend of the screw 84 are both joumaled in a mounting plate 85 at one end of the instrument base 86, and the other end of the screw 84 is journaled in a mounting plate 87 at the other end of the base 86. As the ball screw 84 is rotated by the horizontal drive motor 82, it moves a ball nut 88 threaded onto the screw 84 and carrying a rod 89 which extends upwardly through a block 90 fixed to the rear corner of the frame 81. The nut 88 also carries a carriage 91 along the X-axis defined by the guide rod 96 and the ball screw 84; the carriage 91 in turn is connected to the forward end of the frame 81 via a plate 92 and a bracket 93 to be described in more detail below. Consequently, whenever the stepping motor 82 is energized, the frame 81 is moved along the X-axis in either direction, depending upon the direction of rotation of the reversible motor 82.

As the frame 81 is moved along the X-axis by the horizontal drive assembly, it carries with it a vertical drive assembly mounted on the plate 92 which is attached to and part of the carriage 91. The lower end of the plate92 is secured to a guide block 94 by means of a plurality of machine screws 95. The guide block 94 is provided with ball bushings riding on a guide rod 96 so that the block 94 slides over the rod 96, supporting the vertical drive assembly which in turn carries the frame 81 along the Y-axis. To move the fame 81 along the Y-axis, the

vertical drive assembly on the plate 92 includes a second reversible stepping motor 97 which drives a vertical ball screw 98 via a belt 99. The screw 98, which is journaled in a pair of mounting plates 100 and 101 secured to the plate 92, meshes with a ball nut 102 mounted on the side ofa guide block 103 having ball bushings riding on a vertical guide rod 104 supported by the plates 100 and 101. The guide block 103 in turn is connected to the bracket 93, which in turn carries the frame 81 by a pair of machine screws 105 so as to rigidly couple the frame 81 to the vertical drive assembly. Consequently, whenever the vertical drive motor 97 is energized, the frame 81 is carried either upwardly or downwardly, depending upon the direction of rotation of the motor 97, with the guide block 103 sliding over the guide rod 104, and the block 90 sliding over the rod 89 so that the rods 104 and 89 define the Y-axis.

In accordance with one aspect of the invention, means are provided for automatically stopping the scanning movement of the radiation source and detector assemblies at a preselected stop position, and for automatically stopping the return movement of the assemblies at a preselected start position. Thus, in the illustrative instrument a pair oflimit switches LS1 and LS2 (FIG. 8) are positioned at selected start and stop positions for a scanning traverse, along the Y-axis, and limit switches LS3 and LS4 are positioned at selected start and stop positions for a scanning traverse along the X-axis, As the frame 81 traverses the distance between the two switches LS1 and LS2 during a vertical scan, a cam 110 projecting laterally from the guide block 103 engages the actuator of the switch LS2 at the preselected stop position, thereby automatically reversing the direction of the vertical drive motor 97 to stop the scanning movement of the frame 81 and to start the return movement thereof. As the frame 81 returns to its starting position, the cam 110 engages the actuator of the switch LS1, thereby deenergizing the vertical drive motor 97. The control circuitry for effecting these operations in response to actuation of the switch LS2 will be described below. A similar system is included in the horizontal scan assembly; that is, a cam 111 engages the actuator of the limit switch LS4 when the frame 81 reaches the preselected stop position, thereby reversing the direction of the horizontal drive motor 82 to stop the scanning movement of the source and detector assemblies and to start the return movement thereof. At the start position, the cam 111 engages the actuator of the switch LS3 to stop the return movement ofthe frame 81.

In accordance with another aspect of the invention, means are provided to enable the operator to adjust the aforedescribed start and stop points so that the beam traverse can be started and stopped at the end points of the particular bone section selected for the bone-density measurement. Thus, in the illustrative instrument, the two limit switches LS1 and LS2 in the Y-axis scanning assembly are mounted on a slide bar 112 on the plate 92 so that the switches can be moved to any desired positions along the length of the bar 112. To move the limit switch LS1 or LS2, the operator first depresses a vertical adjust" button B1 located adjacent the vertical scale for the screen 14 on the front of the instrument; if the cam 100 is already located at the start position represented by the switch LS1, for example, the button B1 is automatically illuminated when it is depressed, and the operator then simply depresses either the "up button B2 or the down" button B3 adjacent the button B1 to slide the switch LS1 along the bar 112 to the desired start position for the next scan. If the cam 110 is not aligned with the start position represented by the switch LS1, the button B1 is not illuminated when it is depressed; in this situation, the operator keeps the vertical adjust button Bl depressed and, in addition, depresses either the up button B2 or the down" button B3 to bring the cam 110 into the alignment with the switch LS1, at which point the button B1 is automatically illuminated. Thus, the vertical adjust" button B1 is automatically illuminated whenever l it is depressed and (2) the cam 110 engages the actuator of either switch LS1 and LS2, to indicate to the operator that any further movement of the scanning assembly will efiect an adjustment in the start or stop position represented by the corresponding switch LS1 or LS2.

Turning now to FIGS. 9 and 10 for a more detailed description of the internal mechanism for effecting adjustments of the position of switch LS2 in response to the operator's manipulation of the buttons B1-B3, depression of the button B1 conditions a solenoid 120 for energization when either one of the switches LS1 or LS2 is actuated by the cam 110. For example, when the switch LS2 is actuated, the solenoid 120 is energized to throw a latching pin 121 into engagement with a rack 122 connected to the switch LS2, thereby latching the switch LS2 to the vertical scanning assembly for movement therewith. More particularly, the armature of the solenoid 120 is con nected to the lower end of a latching arm 123 pivotally mounted on the bracket 93, so that energization of the solenoid 120 swings the latching arm 123 about its central pivot point 124 to advance the pin 121 mounted on the opposite end of the arm 123 into engagement with the rack 122. Of course, if the cam is already engaging the actuator of switch LS2 when the vertical adjust" button B1 is depressed, the solenoid is energized immediately upon depression of button B1 to latch the switch LS2 to the bracket 93. lfthe cam 110 is not engaging the actuator of switch LS2 when button B1 is first depressed, the operator depresses button B2 or B3, while keeping button B1 depressed, to bring the cam 110 into engagement with switch LS2 and thereby energize the solenoid 120; that is, the solenoid 120 is simply conditioned for energization until the cam 110 is brought into engagement with the actuator of switch LS2 by depression of button B2 or B3, whereupon the solenoid 120 is immediately energized to latch the switch LS2 to the vertical or Y-axis scanning assembly.

After the pin 121 is thrown into latching engagement with the rack 122, the switch LS2 is moved to the desired stop position by depressing either the up" button B2 or the down" button B3. Depression of either one of these buttons energizes the vertical drive motor 97 to move the frame 81 and the bracket 93 in the selected direction, thereby sliding the switch LS2 along the bar 112. More specifically, depression of button B2 energizes the reversible motor 97 in one direction to move the switch LS1 upwardly, while depression of button B3 energizes the motor 97 in the opposite direction to move the switch LS2 downwardly.

It will be understood that adjustment of the position of the vertical start position switch LS1 is effected in the same manner described above for the vertical stop position switch LS2. The only difference is that the latching pin 121 engages a rack 125 connected to the switch LS1 rather than the rack 122 connected to the switch LS2. In both cases, the latching pin 121 is retracted to its unlatched position by a biasing spring 126 whenever the solenoid 120 is deenergized, i.e., when either button B2 or B3 is not depressed or the cam 110 is not engaging either ofthe limit switches LS1 and LS2.

The arrangement for adjusting the positions of the horizontal or X-axis start and stop position switches LS3 and LS4 is similar to that described above for the vertical or Y-axis switches LS1 and LS2. Thus, a horizontal adjust button B4 located adjacent the horizontal scale on the screen 14 on the front of the instrument conditions a solenoid 130 (FIG. 8) for energization whenever one of the limit switches LS3 or LS4 is actuated by the cam 111. When the solenoid 130 is energized, it throws a latching pin 131 into engagement with a ratchet connected to either switch LS3 and LS4 to latch the respective switch to the horizontal scanning assembly for movement therewith. Energization of the horizontal drive motor 82 t9 move the cam 111 into engagement with the actuators of the switches LS3 and LS4, and to move the switches LS3 and LS4 to the selected start and stop positions while the pin 131 is in its latched position, is effected by means of a right" button B5 and a left" button B6 located on opposite sides of the horizontal adjust" button B4, in the same manner as the up" and down buttons B2 and B3 described previously in connection with the Y-axis adjustments. That is, the button B5 energizes the reversible motor 82 in a first direction to move the cam 111 to the right, while the button B6 energizes the motor 82 in the opposite direction to move the cam 1 11 to the left.

Although the operation of the illustrative instrument should be clear from the description given thus far, it will be helpful to briefly summarize the operation thereof with specific reference to one of the exemplary radiographs. Thus, referring to the os calcis radiograph represented in FIG. 3, a measurement of the density of this bone is typically made through the section A-B. Accordingly, level A-B on the radiograph is located at level 4" on the vertical axis of the scale associated with the screen 14 on the front of the densitometer unit 10, and the vertical adjustment buttons Bl-B3 are used to align the illuminated dot with the 4" level on the screen 14. The horizontal adjustment buttons B4-B6 are then used to set the horizontal stop position at point B by moving the illuminated dot along the 4 level until it reaches the 6.5 position on the horizontal scale. Next, the horizontal start position at point A by returning the illuminated dot along the 4" level until it reaches the L7 position on the horizontal scale. The operator then depresses the horizontal scan" button B8, thereby initiating an automatic horizontal scanning cycle during which the shutter mechanism is opened to expose the patients foot to the radiation beam which starts at point A, advances along the bone section A B until it reaches point B, and then returns to the starting point A. As will be described in more detail below, the scanning assembly is advanced at a relatively slow rate during the scanning traverse, and is then returned from the stop point to the start point at a relatively fast rate.

To limit the movement of the vertical and horizontal scanning assemblies at the ends of the respective traversing ranges, a pair of limit switches S and S16 are mounted on the plates 100 and 101 at opposite ends of the vertical guide rod 104, and a second pair of limit switches S17 and S18 are mounted on the plates 87 and 85 at opposite ends of the horizontal guide rod 96. When either the vertical guide block 103 or the horizontal guide block 94 reaches the end of the maximum traversing range for the corresponding scanning assembly, the end of the block actuates one of the switches SIS-S18 via one of a pair of vertical bumper elements 140, 141 or horizontal bumper elements 142, 143 mounted on the guide rods 104 and 96, respectively. More particularly, each of the bumper elements 140-143 includes a peripheral flange adapted to engage the actuator of the corresponding limit switch to reverse the corresponding drive motor 82 or 97, as will be described in more detail below in connection with the electrical control circuitry for the illustrative instrument.

Turning next to the electronic control system illustrated in FIGS. 12 and 13, after the start and stop positions have been set via the buttons Bl-B6, the operator starts a scan in the selected direction by depressing one of the start scan" buttons B7 or B8 on the front of the unit 11. Assuming that a horizontal or X-axis scan is selected, button B8 is depressed so as to close a switch S6 (FIG. 12a) to trigger a flip-flop FFl. The resulting output signal from the flip-flop FFl is applied via a horizontal control gate GH to a series of horizontal motor control gates Gl-G8 to enable the reversible horizontal drive motor 82 (FIG. 12b). The direction of rotation of the motor 82, which is suitably a Slo-Syn" synchronous DC stepping motor, is determined by the order in which the four windings W1-W4 of the motor are energized by power input pulses from a sequence generator SGl (FIG. 12a). More particularly, if the power input pulses are applied to the windings in the order W1, W2, W3, W4, the motor shaft is driven in a first direction so as to move the scanning assembly along the X-axis to the right as viewed in FIG. 8; if the windings are energized in the reverse order, W4, W3, W2, W1, the motor shaft is driven in the opposite direction so as to move the scanning assembly to the left along the X-axis. To permit energization of the windings Wl-W4 in either sequence, each of the windings, is connected to a different pair of the gates GI-G8, and each of the four output lines 201-204 from the sequence generator SGl is connected to one gate in each of two different pairs. More particularly, line 201 is connected to gate G1 in the W1 pair and gate G8 in the W4 pair, line 202 is connected to gate G3 in the W2 pair and gate G6 in the W3 pair, line 203 is connected to gate 65 in the W3 pair and gate G4 in the W2 pair, and line 204 is connected to gate G7 in the W4 pair and gate G2 in the W1 pair. The sequence generator SGl supplies power input pulses sequentially to the four output lines 20], 202, 203, and 204, in that order, but the order in which these pulses are supplied to the windings Wl-W4 depends on whether the odd" gates G1, G3, G5, and G7 (W1, W2, W3, W4), or the even" gates G2, G4, G6, and G8 (W4, W3, W2, W1), are enabled. Thus, the direction of the motor 82 is controlled by enabling either the odd" gates G1, G3, G5 and G7, to energize the motor for rotation in a first direction, or the even" gates G2, G4, G6 and G8, to energize the motor for rotation in the opposite direction.

The enabling of the odd or even" gates G1-G8 is controlled by a flip-flop FF2. More particularly, whenever the horizontal start scan button B8 is depressed to close the switch S6, a triggering signal is applied to the first input of flipflop FF2 via line 205 to produce an enabling output on output line 206 connected to the odd" gates G1, G3, G5, and G7. In this operative state, the motor 82 is driven in the first direction in response to the power input pulses from the sequence generator 861, thereby advancing the scanning assembly along the X-axis to the right as shown in FIG. 8, and as viewed on the screen 14 on the front of the instrument. The rate of advancement is determined by the rate at which the power input pulses are produced by the sequence generator $61, as will be described in more detail below. i

It will be appreciated from the description given thus far that energization of the horizontal drive motor 82 is controlled by three different enabling inputs supplied to the gates Gl-G8 connected to the motor windings Wl-W4. First, the input from the flip-flop FFl enables all the gates Gl-G8, thereby selecting the horizontal drive motor 82 (as opposed to the vertical drive motor 97) for energization. Secondly, the input from the flip-flop FF2 enables either the odd" gates or the even gates, thereby selecting the particular direction in which the motor 82 is to operate; Thirdly, the power input pulses from the sequence generator SGl enable the selected gates at a prescribed rate, thereby controlling the motor speed.

In order to indicate to the operator when the horizontal drive motor 82 is energized, the enabling signal from the horizontal control gate GH also activates indicator lights L6 and L3 mounted within the horizontal start scan" button B8 and the right adjust" button B5, respectively. In the scan mode, these indicator lights illuminate the buttons B8 and B5 as long as the motor 82 is energized, and are extinguished simultaneously with deenergization of the motor 82.

In accordance with one specific aspect of the invention, the shutter mechanism associated with the radioactive source material is automatically opened in response to the initiation of a scan, and is automatically closed at the end of the scan, before the scanning assembly is returned to its starting position. Thus, in the illustrative system, the same signal that triggers the flip-flop FF 1, in response to closing the switch S6, also triggers a flip-flop FF3; the resulting output signal produced by the flip-flop FF3 on line 210 energizes the shutter solenoid SL1 (FIG. 12b) to open the shutter mechanism. Consequently, the patient is automatically exposed to the radiation beam from the radioactive source in response to actuation of the start scan" button B8. The FF3 output also enables a pair of gates G9 and G10 (FIG. 12a) for supplying input pulses to the sequence generator 861 from a low-frequency pulse generator PGl so that the motor 82 is driven at a relatively slow rate during the scanning traverse.

When the horizontal scanning assembly reaches the stop point selected by the operator via the buttons 84-36, the limit switch LS4 (FIG. 12a) is closed, thereby supplying a triggering signal to the second input to the flip-flop FF2 to remove the enabling signal from line 206 and to produce an enabling signal on the other output line 207. Line 207 is connected to the even horizontal motor control gates G2, G4, G6 and G8 so that the motor windings are energized in the sequence W4, W3, W2, W1, thereby reversing the direction of the motor 82 for returning the horizontal scanning assembly to its start posi tion.

The closing of switch LS4 also supplies a triggering signal to the second input to the flip-flop FF3 to remove the enabling signal from the shutter control line 210, thereby deenergizing the shutter solenoid SL1 to close the shutter mechanism. Thus, the patient is exposed to the radiation beam only during the scanning traverse, and not during the return traverse.

In accordance with one particular aspect of the invention, the automatic control system includes means for returning the scanning assembly from the stop position to the start position at a rate substantially faster than the scanning rate at which the beam is advanced from the start position to the stop position. Thus, in the illustrative system, the triggering of the flipflop FF3 in response to closing of the switch LS4 at the end of a scanning traverse, disables the gates G9 and G10 to disconnect the low-frequency pulse generator PGl from the input to the sequence generator SG1, and enables a gate G11 to connect a high-frequency pulse generator PG2 to the input to the sequence generator SG1. More particularly, the switching of the flip-flop FF3 removes the enabling signal from gates G9 and G10, and supplies an enabling signal via line 211 to the gate G11. In one exemplary embodiment, the high-frequency pulse generator PG2 produces pulses at a rate of 480 pps, while the low-frequency generator PGl produces pulses at a rate of 12 pps, so that the return rate is 40 times as fast as the scanning rate.

When the horizontal scanning assembly is returned to its starting position, the limit switch LS3 (FIG. 12a) is closed, thereby triggering the flip-flop FFl to disable all the gates Gl-G8 associated with the horizontal drive motor 82. This deenergizes the motor 82, and the instrument is thus inactivated until the operator initiates another scanning cycle or adjusts the start or stop positions. The indicator lights L6 and L3 are extinguished simultaneously with the deenergization of the motor 82, to indicate to the operator that the drive motor 82 is deenergized.

When the operator wishes to adjust the horizontal start or stop position, he depresses the horizontal adjust button B4 and either the right adjust" button B5 or the left adjust" button B6, thereby actuating the instrument in an adjust" mode instead of the "scan" mode described previously. For example, if the operator wishes to move the horizontal stop position to the right, he depresses both button B4 and button B5. Deferring the discussion of button B4 for the moment, depression of button B5 moves the cam 111 to the right along the X-axis until it engages the actuator of limit switch LS4. More specifically, when the operator depresses button B5, switch 53A is thrown to the broken-line position shown in FIG. 12a, thereby enabling the horizontal drive motor 82 and activating the indicator light L6 via the gate GH, in the same manner described previously in connection with the output signal from the flip-flop FFl. Throwing switch 53A to the broken-line position also causes a triggering signal to be supplied from a source V1 to the first input of the flip-flop FF2, thereby producing an enabling signal on the output line 206 (in the same manner as the triggering signal from the switch S6 described previously). This enables the odd gates G1, G3, G5 and G7 to energize motor 82 in the direction which moves the cam 111 to the right. The flip-flop FF3 is not triggered, as it is when the start scan" switch S6 is closed, so the sequence generator SG1 receives its input signals from the high-frequency pulse generator PG2 via gate G11. Consequently, it can be seen that the depression of button B6 energizes the horizontal drive motor 82 to move the cam 111 to the right at the same high speed at which the motor 82 is operated during the return portion of the scanning cycle described previously.

Returning now to the function of the horizontal adjust button B4, the depression of this button by the operator opens a switch S12 (FIG. 1241) so as to condition the horizontal latching solenoid SL3 (FIG. 12b) for energization. The latching solenoid SL3, which controls the mechanism for latching the limit switches LS3 and LS4 to the horizontal scanning assembly as described previously, is not energized until the cam 111 is advanced into engagement with the actuator of the limit switch LS4, thereby opening a switch S10 and energizing the solenoid SL3 to latch the switch LS4 to the horizontal scanning assembly. At the same time, a pilot light L12 (FIG. 12b) mounted within the button B4 is energized to indicate to the operator that the horizontal limit switch LS4 is latched to the horizontal scanning assembly. Still assuming that the stop position represented by the switch LS4 is to be moved to the right, the button B5 is still depressed by the operator, so the motor 82 remains energized to move the switch LS4 in the desired direction. When the stop position represented by the switch LS4 reaches the desired position, as indicated by the illuminated dot on the instrument screen 14, the operator releases both buttons B4 and B5, thereby opening switch S12 and returning switch 83A to its original position to deenergize the motor 82 and the solenoid SL3 and extinguish the indicator lights L6 and L12.

To return the horizontal scanning assembly to its starting position, the operator depresses the left adjust" button B6. This throws a switch 54A to the broken-line position shown in FlG. 12a to supply an enabling signal to the horizontal control gate GH in the same manner as the switch S3A associated with the right adjust button B5. To energize the motor 82 in the reverse mode, for moving the scanning assembly to the left, the switch S4A also causes a triggering signal to be supplied from the source V1 to the second input of the flip-flop FF2, thereby producing an enabling signal on the output line 207 to enable the even" gates G2, G4, G6, and G8. The flip-flop FF3 is not triggered, as it is when the "start scan" switch S6 is closed, so the sequence generator SG1 still receives its input signals from the high-frequency pulse generator PG2 via gate G11 to energize the motor 82 at the same high speed used during the return portion of the scanning cycle. Of course, when the button B6 is released, the enabling signal is removed from the horizontal control gate GH, and the motor 82 is deenergized.

If the operator wishes to adjust the horizontal starting position, he depresses the horizontal adjust" button B4 until the cam 111 engages the actuator of the limit switch LS3. As described previously in connection with the adjustment of the horizontal stop position, the depression of button B4 opens the switch S12 to condition the horizontal latching solenoid SL3 for energization so that when the cam 111 actuates switch LS3, the solenoid SL3 is energized to latch the switch LS3 to the horizontal scanning assembly. More particularly, actuation of switch LS3 opens a switch S9 to energize the solenoid SL3. At the same time, the pilot light L12 is energized to illuminate the button B4 to indicate to the operator that any further movement of the scanning assembly effects an adjustment of the position of the switch LS3, thereby adjusting the horizontal start position.

For the purpose of selectively activating the indicator lights L3 and L4 within the adjust buttons B5 and B6, respectively, to indicate the direction of movement of the horizontal scanning assembly during an adjustment operation, the buttons B5 and B6 control a pair of switches 53B and S48 (FIG. 12b) associated with the two lights L3 and L4, respectively. More particularly, when button B5 is depressed, it throws the switch 838 to the right to activate the light L3 via a voltage source V2; when button B6 is depressed, it throws the switch 84B to the right to activate the light L4. Consequently, it can be seen that the depression of either button B5 or B6 activates the particular indicator light within that button to provide a visible indication of the selected direction of adjustment.

Turning next to the control system for vertical or Y-axis movement of the scanning assembly, to select a vertical or Y- axis scan the vertical start scan" button B7 isdepressed so as to close a switch S (FIG. 12a) to trigger a flip-flop FF4. The

resulting output signal from the flip-flop FF4 is applied via a vertical control gate GV to a series of vertical motor control gates G2l-G28 to enable the reversible vertical drive motor 97 (FIG. 12b). The direction of rotation of the motor 97 is determined by the order in which the four windings W5-W8 of the motor are energized by power input pulses from the sequence generator SG1. More particularly, if the power input pulses are applied to the windings in the order W5, W6, W7, W8, the motor shaft is driven in a-first direction so as to move the scanning assembly upwardly along the Y-axis; if the windings are energized in the reverse order, W8, W7, W6, W5, the motor shaft is driven in the opposite direction so as to move the scanning assembly downwardly along the Y-axis. To permit energization of the windings W5-W8 in either sequence, each of the windings is connected to a different pair of the gates G21-G28, and each of the four output lines 201-204 from the sequence generator SG1 is connected to one gate in each of two different pairs. More particularly, line 201 is connected to gate G21 in the W5 pair and gate G28 in the W8 pair, line 202 is connected to gate G23 in the W6 pair and gate G26 in the W7 pair, line 203 is connected to gate G25 in the W7 pair and gate G24 in the W6 pair, and line 204 is connected to gate G27 in the W8 pair and gate G22 in the W5 pair. The sequence generator SG1 supplies power input pulses sequentially to the four output lines 201, 202, 203, and 204, in that order, but the order in which these pulses are supplied to the windings W5W8 depends on whether the odd gates G21, G23, G25, and G27 (W5, W6, W7, W8), or the even gates G22, G24, G26, and G28 (W8, W7, W6, W5), are enabled. Thus, the direction of the motor 97 is controlled by enabling either the odd gates G21, G23, G25, and G27, to energize the motor for rotation in a first direction; or the even" gates G22, G24, G26, and G28, to energize the motor for rotation in the opposite direction.

The enabling ofthe odd or even gates G21-G28 is controlled by a flip-flop FFS. More particularly, whenever the vertical start scan button B7 is depressed to close the switch $5; a triggering signal is applied to the first input of flip-flop FF5 via line 225 to produce an enabling output signal on output line 226 connected to the odd gates G21, G23, G25, and G27. In this operative state, the motor 97 raises the vertical scanning assembly in response to the power input pulses from the sequence generator SG1, with the rate of advancement determined by the rate at which the power input pulses are generated.

It will be appreciated from the description given thus far that energization of the vertical drive motor 97 is controlled by three different enabling inputs supplied to the gates G21-G28 connected to the motor windings W5-W8. First, the input from the flip-flop FF4 enables all the gates G21-G28, thereby selecting the vertical drive motor 97 (as opposed to the horizontal drive motor 82) for energization. Secondly, the input from the flip-flop F F5 enables either the odd" gates or the even gates, thereby selecting the particular direction in which the motor 97 is to operate. Thirdly, the power input pulses from the sequence generator SG1 enable the selected gates at a prescribed rate, thereby controlling the motor speed.

In order to indicate to the operator when the vertical drive motor 97 is energized, the enabling signal from the vertical control gate GV also activates indicator lights L5 and L1 mounted within the vertical start scan button B7 and the up adjust button B2, respectively. In the scan mode, these indicator lights illuminate the buttons B7 and B2, respectively, as long as the motor 97 is energized, and are extinguished simultaneously with deenergization of the motor 97.

In keeping with the invention, the same signal that triggers the flip-flopFF4, in response to closing of the switch S5, also triggers the flip-flop FF3 to automatically open the shutter mechanism in response to the initiation of a vertical scan. More particularly, the resulting output signal produced by the flip-flop FF3 on line 210 energizes the shutter solenoid SL1 to open the shutter mechanism. As described previously, the F F3 output also enables the gates G9 and G10 for supplying input pulses to the sequence generator SG1 from the low-frequency pulse generator PG]. When the scanning assembly reaches the vertical stop point selected by the operator via the buttons 81-83, the limit switch LS2 (FIG. 12a) is closed, thereby supplying a triggering signal to the second input to the flip-flop FF3 to remove,the enabling signal from the shutter control line 210, and thus deenergize the shutter solenoid SL1 to close the shutter mechanism. Thus, the patient is exposed to the radiation beam only during the scanning traverse, and not during the return traverse.

The closing of switch LS2 also supplies a triggering signal to the second signal to the flip-flop FFS to remove the enabling signal from line 226 and to produce an enabling signal on the other output line 227. Line 227 is connected to the "even vertical motor control gates G22, G24, G26 and G28 so that the motor windings are energized in the sequence W0, W7, W6, W5, thereby reversing the direction of the motor 97 for returning the vertical scanning assembly to its start position.

Finally, the triggering of flip-flop FF3 causes the scanning assembly to be returned from the stop position to the start position at a rate substantially faster than the scanning rate at which the beam is advanced from the start position to the stop position, in the same manner described previously for the horizontal scanning assembly. When the vertical scanning assembly is returned to its starting position, the limit switch LS1 (FIG. 12a) is closed, thereby triggering the flip-flop FF4 to disable all the gates G21-G28 associated with the horizontal drive motor 97. This deenergizes the motor 97, and the instrument is thus inactivated until the operator initiates another scanning cycle or adjusts the start or stop positions. The indicator lights L5 and L1 are extinguished simultaneously with the deenergization of the motor 97.

When the operator wishes to adjust the vertical start or stop position, he depresses the vertical adjust" button B1 and either the up adjust button B2 or the down adjust button B3. Turning first to the function of the buttons B2 and B3, the depression of one of these buttons brings the cam into engagement with the appropriate limit switch LS1 or LS2. For example, if the operator wishes to raise the vertical stop position, he depresses both button B1 and button B2. Deferring the discussion of Button Bl for the moment, depression of button B2 moves the cam 110 upwardly until it engages the actuator of limit switch LS2. More specifically, when the operator depresses button B2, switch 51A is thrown to the broken-line position shown in FIG. 12a, thereby enabling the vertical drive motor 97 and activating the indicator light L5 via the gate GV, in the same manner described previously in connection with the output signal from the flip-flop F F4. Throwing switch 81A to the broken-line position also causes a triggering signal to be supplied from a source V1 to the first input of the flip-flop FFS, thereby producing an enabling signal on the output line 226 (in the same manner as the triggering signal from the switch S5 described previously). This enables the odd gates G21, G23, G25 and G27 to energize motor 97 to raise the cam 110. The flip-flop FF3 is not triggered, as it is when the start scan" switch S5 is closed, so the sequence generator SG1 receives its input signals from the high-frequency pulse generator PG2 via gate Gll. Consequently, it can be seen that the depression of button B2 energizes the vertical drive motor 97 to raise the cam 110 at the same high speed at which the motor 97 is operated during the return portion of the scanning cycle described previously.

Returning now to the function of the vertical adjust button Bl, the depression of this button by the operator opens a switch S13 (FIG. 12a) so as to condition the vertical latching solenoid SL2 (FIG. 12b) for energization. The latching solenoid SL2, which controls the mechanism for latching the limit switches LS1 and LS2 to the scanning assembly as described previously, is not energized until the cam 110 is advanced into engagement with the actuator of the limit switch LS2, thereby opening a switch S7 and energizing the solenoid SL2 to latch the switch LS2 to the scanning assembly. At the same time, a ilot light L13 (FIG. 12b) mounted within the button B1 is energized to indicate to the operator that the vertical limit switch LS2 is latched to the vertical scanning assembly. Still assuming that the stop position represented by the switch LS2 is to be moved upwardly, the button B2 is still depressed by the operator, so the motor 97 remains energized to move the switch LS2 in the desired direction. When the stop position represented by the switch LS2 reaches the desired position, as indicated by the illuminated dot on the instrument screen 14, the operator releases both buttons B1 and B2, thereby deenergizing the motor 97 and the solenoid SL2 and extinguishing the indicator lights L5 and L13.

To return the vertical scanning assembly to its starting position, the operator depresses the down adjust button B3. This throws a switch 82A to the broken-line position shown in FIG. 12a to supply an enabling signal to the vertical control gate GV in the same manner as the switch SlA associated with the up adjust button B2. To energize the motor 97 in the reverse mode, for lowering the scanning assembly, the switch S2A also causes a triggering signal to be supplied from the source V1 to the second input of the flip-flop FFS, thereby producing an enabling signal on the output line 227 to enable the even gates G22, G24, G26, and G28. The flip-flop FF3 is not triggered, as it is when the start scan switch 35 is closed, so the sequence generator SGl still receives its input signals from the high-frequency pulse generator PG2 via gate G11 to energize the motor 97 at the same high speed used during the return portion of the scanning cycle. Of course, when the button B3 is released, the enabling signal is removed from the vertical control gate GV, and the motor 97 is deenergized.

If the operator wishes to adjust the vertical starting position, he depresses the "vertical adjust" button B1 until the cam 110 engages the actuator of the limit switch LS1. As described previously in connection with the adjustment of the vertical stop position, the depression of button B1 opens the switch $13 to condition the vertical latching solenoid SL2 for energization so that when the cam 110 actuates switch LS1, the solenoid SL2 is energized to latch the switch LS1 to the vertical scanning assembly. More particularly, actuation of switch LS1 opens a switch 58 to energize the solenoid SL2. At the same time, the pilot light L13 is energized to illuminate the button B1 to indicate to the operator that any further movement of the scanning assembly efiects an adjustment of the position of the switch LS1, thereby adjusting the vertical start position. I

For the purpose of selectively activating the indicator lights L1 and L2 within the adjust buttons B2 and B3, respectively, to indicate the direction of movement of the vertical scanning assembly during an adjustment operation, the buttons B2 and B3 control a pair of switches 51B and 52B (FIG. 12b) associated with the two lights L1 and L2 respectively. More particularly, when button B2 is depressed, it throws the switch 813 to the right to activate the light L1 via a voltage source V2; when button B3 is depressed, it throws the switch 528 to the right to activate the light L2. Consequently, it can be seen that the depression of either button B2 or B3 activates the particular indicator light within that button to provide a visible indication of the selected direction of adjustment.

Regardless of whether the illustrative control system is operating in the vertical or horizontal scanning mode, the opening of the shutter mechanism in response to the triggering of flip-flop FF3 actuates a switch 56 (FIG. 8 and 12a) to energize an indicator light L14 (FIG. 12a) within the count" button B10, thereby illuminating the button B to indicate to the operator that a count is in progress. When the switch is returned to its moral position in response to closing of the shutter at the end ofa scanning traverse, it supplies a disabling signal to a gate G12 (FIG. 12a). While the shutter is open, the switch 56 causes an enabling signal to be supplied to the gate G12, but the gate is disabled by the same signal from flip-flop FF3 that disables the gate G11. Consequently, it can be seen that during normal operation of the instrument, the gate G12 is always disabled. However, in the event that the shutter remains open after a scanning traverse is completed, the gate G12 receives enabling signals from both the flip-flop FF3 and the switch 56, thereby transmitting low-frequency pulses, e.g., 3 pps, from a frequency divider D1 to pulse an indicator light L11 alternately on and off. The light L11 is mounted within the stop" button B11 on the front of the unit 11, so that the pulsing thereof produces flashing illumination of the button Bll to indicate to the operator that a malfunction has occurred in the shutter mechanism. The operator can then take the necessary precautions to avoid prolonged exposure to the radiation beam until the shutter mechanism is repaired.

To insure that only one of the scanning assemblies is opera tive, and in only one selected mode, at any given time, the il lustrative control system includes certain redundancies to disable certain portions of the system when another portion thereofis enabled. Thus, the output lines from the two control gates GH and GV are interconnected via a pair of gates G30 and G31 (FIG. 12a) which respond to an output signal from either gate GH or CV to disable the system associated with the other gate. More specifically, if an enabling signal is transmitted from the horizontal control gate GH to enable the horizontal motor control gates G1-G8, the gate G30 is enabled to supply a disabling signal to the vertical motor-control system. Conversely, if an enabling signal is transmitted from the vertical control gate GV to enable the vertical motor control gates G2l-G28, the gate G31 is enabled to supply a disabling signal to the horizontal motor-control system.

For the purpose of preventing actuation of two different adjustment circuits at any given time, each of the adjust switches Sla, 82a, 53a and 84a is associated with means for disabling the other three adjustment circuits. For example, when switch S1 1 is thrown to the broken-line position shown in FIG. 12a a signal is transmitted via an inverter I19 and line 230 to trigger the flip-flop FFS to produce an output signal that enables the odd" vertical motor control gates, thereby energizing the vertical drive motor 97 to raise the scanning assembly. At the same time, signals transmitted via inverters l7, l8, and I9 disable the adjust circuits associated with the other three adjust switches S211, S311, and $40 so that only one adjust circuit can be actuated at any given time. More particularly, a signal transmitted via inverter 17 and line 231 disables the adjustment circuit associated with switch 53a, the signal transmitted via inverter T8 and line 232 disables the adjustment circuit associated with switch S20, and the signal transmitted via inverter l9 and line 233 disables the adjustment circuit associated with switch S4a.

Similarly, when switch 820 is shown to the broken-line position shown in FIG. 12a, a signal is transmitted via inverter I20 and line 234 to trigger the flip-flop FF5 to produce an output signal which enables the even" vertical motor control gates and thereby energize the vertical drive motor 97 to lower the scanning assembly. Signals transmitted via inverters I10, I11, and 112 in response to the switching of 52a disable the other three adjustment circuits. Switch 53a operates in a similar manner through inverters I21, and I13, I14, and 115, and switch 84a operates through inverters I22 and 116, I17, and 118.

For the purpose of insuring that the adjustment system is disabled when either a horizontal or vertical scan is in progress, both the output from the flip-flop FFl which enables the horizontal drive motor, and the output from the flip-flop FF4 which enables the vertical drive motor, are connected to a gate G32. The presence of either one of these signals at the gate G32 produces a signal which disables the entire adjustment system via inverters ll, I2, l3, l4, l5, and 16. More specifically, the signals transmitted through the four inverts 11 through 14 disable the adjustment circuits associated with the four adjust switches Sla, 82a, 53a, and 84a respectively, while the signals transmitted via the inverters l5 and I6 prevent energization of the two latching solenoids SL3 and SL2, respectively, so that the latching mechanism cannot be actuated when the instrument is operating in the scan mode.

Tolpermit the operator to stop any. scanafter. it. has been in' 'itiated but before the'completion thereof, afstop" button B11 is provided on. thefront. of the unit 111. When. the operator depresses the button B11, a switch-S11 isactuatedto supply triggering signals to the-same inputs to thefiip-flops FF 1 and FF4 to which. the. respective start position switches LS3 and LS1 are connected. Consequently, the. depression of button B11 triggers whichever one of theflip-flops FF]; and FF4 that is supplyingv an enabling signal to gaterGl-l or GV, to remove such enabling signal from the gate and thereby deenergize the corresponding motor 82.or 97. Thus, the scanning assembly is simply stopped at the position where it is locatedat the time the stop" button B11 is depressed, so that the: scan can be subsequently resumed, if desired, by simply depressing the appropriate start scan button B7 or B8. The stop button B11 may also be used to stop a stationary count in the event that the instrument is used for that purpose.

The illustrative system also includes means to enable the operator to select either of: two different scanning speeds. As described previously, the normal. scanning rate is controlled by pulses fed to the sequence generator SGI from. the lowfrequency pulse generator PGl via gate G9 at a rate of 6 pps. However, if the operator depresses a' 2- -speed" button B9 on the front of the unit 11, a switch S19 is actuated to disable the gate G9 and enable a gate G10, thereby bypassing the. divider D2 so that pulses are fed directly from the generator PGl to the sequence generator 561 at a rate of 12 pps. Consequently, the scanning speed is increased by a factor of two. To provide a visible indication that the higher scanning speed has been selected, actuation of the switch S19 also illuminates an indicator light L15 mounted within. the button B9.

In order to limit the maximum displacement of the scanning assembly, a first pair of maximum limit switches S15. and S16 (FIGS. 8 and 12a) are mounted at opposite ends of the guide rod 104, and a second pair of limit switches S17 andS18 are mounted at opposite ends of the guide rod 96. When the remove the reset signaland. thereby enable the scaler 301. Thus, the scaler. 301 is enabled to count the pulses from gate G50. each time a scanningoperation is initiated.

In accordance with one important aspect of the present invention, buffer storage means are operatively connected to the scaler'or other counting device for periodically receiving the count accumulated in the scaler at repetitiveintervals during each scanning cycle, after which the data is read out of the buffer storage whilethe scaler resumes counting of the pulses from gate G50; Thus, in the illustrative embodiment. a buffer storage 302; is. operatively connected to the scaler 301 for receiving data from the scaler 301 at successive intervals during a; scanning cycle. With this arrangement, the data accumulated in the scaler 301 can be rapidly transferred from the scaler 301 into. the, buffer storage 302 during periodic data transfer intervals of short duration, e.g., 20. microseconds, so that the counting operation carried out by the scaler 300 is interrupted for only a very short interval. Consequently, even though the scanning operation is continuous, and the data accumulated in the scaler 301 is transferred out of the scaler at regular intervals while the scanning operation is. in progress, only. an extremely small percentage of the data represented by the electrical output signals from the detector assembly 70 is lost during any given scanning cycle. After the data is transferred into the buffer storage 302, it is read out of the buffer storage at the relatively slow rate required by the readout system, while the scaler resumes the counting operation. In a typical operation,'the scaler 301 counts for 30 seconds, is interrupted for a 20-microsecond interval during which the data accumulated in the scaler is transferred into the buffer storage 302, is reset, and then resumes counting for another 30- second period while the transferred data is read out of the buffer storage 302.

In keeping with the invention, an automatic control system is operatively associated with the scaler and buffer storage units for automatically interrupting the counting operation at periodic intervals during each scanning traverse, transferring the accumulated count from the scaler to the buffer storage, resetting the scaler, and then resuming the counting. Thus, in the illustrative system, a timer 310 is actuated each time a scanning cycle is initiated, and the timer 310 then automatithat the odd" gates G1, G3, G5, and G7 are again enabled by the output from flip-flop FF2 so that the motor is again reversed. Consequently, it can be seen that the motor is continually reversed, with the horizontal-scanning assembly'oscillating accordingly, until the operator presses the button B6 to return the scanning assembly to the left of the originally selected stop point. The other three maximum limit switches S15, S16, and S17 operate in the same manner, S15 controlling the odd" vertical gates G21G27, S16 controlling the even" vertical gates 622-028, and S17 controlling the "even" horizontal gates G2-G8.

it will be recalled that the intensity of the attenuated radiation beam transmitted through the selected bone section is de tected by a detector assembly 70 which converts the detected radiation into light scintillations. These light scintillations are generally proportional in photon energy to the energy of the disintegrations which cause them, and are converted into corresponding voltage or current pulses proportional to both the light scintillations and the disintegrations which cause them. Such pulses are then discriminated on the basis of their amplitude, and counted.

Turning now to H6. 13, the pulses produced by the detector assembly 70 are transmitted via line 300 and a gate G to a scaler 301 which counts the pulses and stores the accumulated count in units, 10s, l00s, 1,000s, 10,000s, and 100,000s, Before a scanning cycle is initiated, the scaler 301 is held in its reset state; but each time a scanning cycle is initiated, the same signal which energizes the shutter solenoid SL1 via line 210 is transmitted via line 303 to a gate G52 to .cally produces output pulses at regular intervals, e.g., 30- second intervals, throughout the scanning traverse. Each time an output pulse is produced by the timer 310, it triggers a flipflop FF6 which in turn enables a gate G51 to transmit clock pulses from a clock pulse source 311, e.g., a high-speed oscillator, to a shift register programmer 312. in response to the clock pulses, the shift registerprogrammer 312 produces a succession of output pulses; the first of which is produced on line 313 to disable the gate G50 and thereby interrupt the transmission of pulses to the scaler 301. The second output signal generated by the shift register programmer 312 is transmitted via line 314 to the buffer storage 302 for the purpose of strobing the data accumulated in the scaler 301 into the buffer storage 302. The third output signal is transmitted via line 315 to the gate G52 for the purpose of resetting the scaler 301 to zero before resuming the counting operation. Finally, the fourth output signal generated by the shift register programmer 312 resets the programmer itself via reset line 316.

Thus, it can be seen that each time the timer 310 generates an output pulse, the control system automatically interrupts the counting by the scaler 301, strobes the accumulated count into the buffer storage 302, and resets both the scaler 301 and the shift register programmer 312. The same signal which resets the scaler 301 via gate G52 also resets the timer 310 to zero time via reset line 317. The resetting of the timer 310 in turn resets the flip-flop FF6, thereby removing the enabling signal from the gate G51 to terminate the flow of clock pulses to the shift register programmer 312.

1n keeping with the present invention, the automatic control system also includes readout control means for automatically reading data out of the buffer storage during the counting period following each data transfer interval during which the data accumulated in the scaler is transferred to the buffer storage. Thus, at the end of each 20-microsecond data transfer interval, the same signal which resets the scaler 301 via gate G52 is applied via line 320 to a tape punch motor actuator 321 which in turn produces an output signal for starting a tape punch motor (in the event that a punched tape output is utilized). The output signal from the actuator 321 also enables a gate G53 so as to transmit pulses derived from one ofthe pulse generators PGl or PG2, e.g., 3 pps pulses from divider D1, through delay means 322 to enable a gate G54. When the gate G54 is enabled, pulses derived from one of the pulse generators PGl or PG2, e.g., l2 pps pulses from PG], are supplied to the first stage of a conventional ripple counter 323. The purpose of the delay means 322 is to delay the start of the ripple counter 320 until the tape punch motor has been brought up to speed.

In the illustrative system of FIG. 13, the ripple counter 323 includes four stages, so that eight output signals are available at any given time; in FIG. 13, these eight available outputs have been designated A and A for the first stage, B and Ffor the second stage, C and Cfor the third stage, and D and Dfor the fourth stage. It will be understood that the specific combination of output signals produced changes each time a pulse is fed to the ripple counter 323 from the delay network 322, and these different combinations of output signals are utilized to enable different readout gates associated with the various units of the buffer storage 302. In the exemplary embodiment, the buffer storage 302 is illustrated as having six different units for storing data representing the units, 's, l00s, 1,000s, 10,000s, and 100,000's of the counts accumulated in the scaler 301. Thus, different combinations of the output signals available from the ripple counter 323 are supplied to six different gates G61, G62, G63, G64, G65 and G66. Each of the gates G61 through G66 in turn supplies enabling signals to one of six four-gate groups G71a-G7ld, G72a-G72d, G73a-G73d, G74aG74d, G75aG75d, and G76a-G76d, respectively, for reading data out of the six corresponding units of the buffer storage 302. As will be apparent from the ensuing description, the different combinations of output signals which enable the gates G61 through G66 enable the gates sequentially so that the data stored in the six units of the buffer storage 302 is read out sequentially.

For purposes of clarity, the various inputs to the gates G61-G66 have been designated by the symbols A, A, B,, C, C, D, D in FIG. 3, it being understood that each of these symbols represents one of the outputs from the ripple counter 323. Thus, gate G61 is enabled when output signals are present atK, B C, and D; gate G62 is enabled by signals A B, C, and D; gate G63 by signals A, E C, and D; gate G64 by signals A, B, C, and D; gate G65 by signals A,, C, and D; and gate G66 by signals A B, C, and D. When gate G61 is enabled, it in turn enables gates G71a-G71d so that the units data in the buffer storage 302 is transmitted via lines 330, 331, 332, and 333 to a conventional BCD-to-decimal converter 334 (assuming the data is stored in BCD form in the buffer storage). The converter 334 in turn supplies enabling signals to a conventional printer, tape punch or the like for producing the final output indicated at in FIG. 1. When gate G62 is enabled, it enables gates G72a-G72d to transmit the 10's data from the buffer storage 302 via lines 330-333 to the converter 334, and gates G63-G66 enable gates G73a-G73d, G74a-G 74d, G75a-G75d, and G76a-G76d in the same manner to sequentially transmit the 100's, 1,000's, 10,000s, and 100,0005 data to the converter 334.

After all the data has been read out of the buffer storage 302, a signal is transmitted from the D output of the ripple counter 323 via line 340 to reset both the tape punch motor actuator 321 and the delay means 322. The readout system is then in condition for another data transfer cycle in response to the next output signal from the timer 310.

As can be seen from the foregoing detailed description, this invention provides an improved bone density measuring instrument which automatically scans a preselected bone section with a radiation beam, and provides the maximum amount of information on the density of the bone throughout the particular section that is scanned. The illuminated dot on the screen on the front of the instrument can be correlated with corresponding scales superimposed on a radiograph of the selected bone section to facilitate location of the scanning beam at the precise bone section selected for the density measurement. Furthermore, because of the rapid return rate provided for the scanning assembly, the time required for each scanning cycle is minimized so as to permit a greater number of measurements to be made with the instrument in any given time period. The improved radiograph template permits two different radiographs of the selected bone to be made from two difierent positions on a single radiograph film, with one of the radiographs being used to locate the particular section of bone to be scanned, and the other radiograph being used to determine the thickness of the bone section.

We claim as our invention:

1. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam through a selected section of a particular bone, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output signal representing the intensity of the detected beam, means for moving said scanning assembly back and forth along a limited predetermined path relative to said bone so as to scan a predetermined portion of the bone thereby varying said electrical output signal in accordance with variations in the density of the scanned portion of the bone, means for stopping said scanning assembly at the opposite end positions of said preselected limited path, and means comprising a visible light source moving in synchronism with said scanning assembly for displaying a visible light spot on a grid for providing a continuous visual indication of the position of said scanning assembly relative to the particular bone being examined to allow accurate scanning path selection.

2. Apparatus for measuring the density of bones comprising the combination of a scanning assembly which includes a source of radioactive material, a shutter mechanism associated with said source for controlling the transmission of radiation emitted by said source, means for forming a radioactive beam from said radioactive source, means for transmitting said radiation beam through a selected section of a particular bone, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output signal representing the intensity of the detected beam, means for moving said scanning assembly back and forth along a limited predetermined path relative to said bone so as to scan a predetermined portion of the bone thereby varying said electrical output signal in accordance with variations in the density of the scanned portion of the bone, means for stopping said scanning assembly at the opposite end positions of said preselected limited path, means for automatically opening said shutter mechanism in response to the initiation of a scan to expose the selected bone section to a radiation beam, and means for automatically closing said shutter mechanism in response to the arrival of said scanning assembly at said preselected stop position.

3. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam through a selected bone section, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output pulse representing the intensity of the detected beam, means for moving said scanning assembly along a predetermined path relative to said bone so as to scan said selected bone section, thereby varying said electrical output pulses in accordance with variations in the density of the selected bone section, counting means operatively connected to said detecting means for counting said output pulses, buffer storage means operatively connected to said counting means for periodically receiving the count accumulated in said counting means, and automatic control means for periodically interrupting the transmission of said pulses to said counting means, transferring, the count accumulated in said counting means to said buffer storage means, resetting said counting means, and then resuming the transmission of said pulses to said counting means.

4. Apparatus as set forth in claim 3 wherein said counting means is a scaler.

5. Apparatus as set forth in claim 3 wherein said automatic control means includes a timer for producing control signals at regular intervals, means for actuating said timer in response to the initiation of said scanning movement of said scanning assembly, and means responsive to said control signals produced by said timer for interrupting the transmission of said pulses to said counting means for a brief count transfer interval.

6. Apparatus as set forth in claim wherein said automatic control means includes programmer means operatively connected to said timer for sequentially l) interrupting the transmission of said pulses to said counting means, (2) transferring the count accumulated in said counting means to said buffer storage means, (3) resetting said counting means, (4)

resetting said timer to start a new timing period, and (5) resuming the transmission of said pulses to said counting means to start a new counting period.

7. Apparatus as set forth in claim 6 which includes a source of clock pulses and gate means responsive to said control signals from said timer for transmitting said clock pulses to said programmer means to actuate said programmer means, said gate means also being responsive to the resetting of said timer to stop the transmission of said clock pulses to said programmer means to deactuate said programmer means.

8. Apparatus as set forth in claim 3 which includes readout control means operatively connected to said buffer storage means and said automatic control means for reading the count out of said buffer storage means each time the transmission of said pulses to said counting means is resumed.

9. Apparatus as set forth in claim 8 wherein said buffer storage means includes multiple storage units for different bits of data representing the counts transferred to said bufi'er storage means from said counting means, and said readout control means includes a ripple counter, means for supplying input pulses to said ripple counter for sequentially producing different combinations of output signals, and a plurality of gates connected between said ripple counter and said bufler storage means with each gate being responsive to a different combination of said output signals for sequentially reading out the different bits of data from the multiple storage units of said buffer storage means.

10. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam'through a selected bone section, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing electrical output pulses representing the intensity of the detected beam, means for moving said scanning assembly along a predetermined path relative to said bone so as to scan said selected bone section, thereby varying said electrical output pulses in accordance with variations in the density of the selected bone section, counting means operatively connected to said detecting means for counting said output pulses, buffer storage means operatively connected to said counting means for periodically receiving the count accumulated in said counting means, data transfer control means for interrupting the transmission of said output pulses to said counting means during repetitive data transfer intervals and transferring the count accumulated in said counting means to said buffer storage means during said transfer intervals, and readout control means for reading out the count stored in said buffer storage means during the readout intervals between said data transfer intervals. 

1. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam through a selected section of a particular bone, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output signal representing the intensity of the detected beam, means for moving said scanning assembly back and forth along a limited predetermined path relative to said bone so as to scan a predetermined portion of the bone thereby varying said electrical output signal in accordance with variations in the density of the scanned portion of the bone, means for stopping said scanning assembly at the opposite end positions of said preselected limited path, and means comprising a visible light source moving in synchronism with said scanning assembly for displaying a visible light spot on a grid for providing a continuous visual indication of the position of said scanning assembly relative to the particular bone being examined to allow accurate scanning path selection.
 2. Apparatus for measuring the density of bones comprising the combination of a scanning assembly which includes a source of radioactive material, a shutter mechanism associated with said source for controlling the transmission of radiation emitted by said source, means for forming a radioactive beam from said radioactive source, means for transmitting said radiation beam through a selected section of a particular bone, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output signal representing the intensity of the detected beam, means for moving said scanning assembly back and forth along a limited predetermined path relative to said bone so as to scan a predetermined portion of the bone thereby varying said electrical output signal in accordance with variations in the density of the scanned portion of the bone, means for stopping said scanning assembly at the opposite end positions of said preselected limited path, means for automatically opening said shutter mechanism in response to the initiation of a scan to expose the selected bone section to a radiation beam, and means for automatically closing said shutter mechanism in response to the arrival of said scanning assembly at said preselected stop position.
 3. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam through a selected bone section, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing an electrical output pulse representing the intensity of the detected beam, means for moving said scanning assembly along a predetermined path relative to said bone so as to scan said selected bone section, thereby varying said electrical output pulses in accordance with variations in the density of the selected bone section, counting means operatively connected to said detecting means for counting said output pulses, buffer storage means operatively connected to said counting means for periodically receiving the count accumulated in said counting means, and automatic control means for periodically interrupting the transmission of said pulses to said counting means, transferring, the count accumulated in said counting means to said buffer storage means, resetting said countiNg means, and then resuming the transmission of said pulses to said counting means.
 4. Apparatus as set forth in claim 3 wherein said counting means is a scaler.
 5. Apparatus as set forth in claim 3 wherein said automatic control means includes a timer for producing control signals at regular intervals, means for actuating said timer in response to the initiation of said scanning movement of said scanning assembly, and means responsive to said control signals produced by said timer for interrupting the transmission of said pulses to said counting means for a brief count transfer interval.
 6. Apparatus as set forth in claim 5 wherein said automatic control means includes programmer means operatively connected to said timer for sequentially (1) interrupting the transmission of said pulses to said counting means, (2) transferring the count accumulated in said counting means to said buffer storage means, (3) resetting said counting means, (4) resetting said timer to start a new timing period, and (5) resuming the transmission of said pulses to said counting means to start a new counting period.
 7. Apparatus as set forth in claim 6 which includes a source of clock pulses and gate means responsive to said control signals from said timer for transmitting said clock pulses to said programmer means to actuate said programmer means, said gate means also being responsive to the resetting of said timer to stop the transmission of said clock pulses to said programmer means to deactuate said programmer means.
 8. Apparatus as set forth in claim 3 which includes readout control means operatively connected to said buffer storage means and said automatic control means for reading the count out of said buffer storage means each time the transmission of said pulses to said counting means is resumed.
 9. Apparatus as set forth in claim 8 wherein said buffer storage means includes multiple storage units for different bits of data representing the counts transferred to said buffer storage means from said counting means, and said readout control means includes a ripple counter, means for supplying input pulses to said ripple counter for sequentially producing different combinations of output signals, and a plurality of gates connected between said ripple counter and said buffer storage means with each gate being responsive to a different combination of said output signals for sequentially reading out the different bits of data from the multiple storage units of said buffer storage means.
 10. Apparatus for measuring the density of bones comprising the combination of a scanning assembly including means for transmitting a radiation beam through a selected bone section, means for detecting the attenuated radiation beam transmitted through said selected bone section and producing electrical output pulses representing the intensity of the detected beam, means for moving said scanning assembly along a predetermined path relative to said bone so as to scan said selected bone section, thereby varying said electrical output pulses in accordance with variations in the density of the selected bone section, counting means operatively connected to said detecting means for counting said output pulses, buffer storage means operatively connected to said counting means for periodically receiving the count accumulated in said counting means, data transfer control means for interrupting the transmission of said output pulses to said counting means during repetitive data transfer intervals and transferring the count accumulated in said counting means to said buffer storage means during said transfer intervals, and readout control means for reading out the count stored in said buffer storage means during the readout intervals between said data transfer intervals. 